The present invention relates to methods of cleaning semiconductor production equipment and particularly to cleaning a chemical vapor deposition chamber.
During a semiconductor device fabrication process, chemical vapor deposition (CVD) based on a gaseous reaction is commonly used to deposit a film on the surface of a semiconductor wafer. During the CVD process, reaction products and byproducts deposit not only on the wafer surface but also on the inner surfaces of the chamber. The undesirable deposit on the chamber inner surfaces may adversely affect the quality of the film deposited on the wafer. For example, particles may separate from the inner surfaces of the chamber and contaminate the film, reducing device yield. The deposit may include silicon, oxygen, nitrogen, carbon, and other species, depending on the film deposition chemistry.
Currently, a cleaning method based on plasma etching using a halogen gas (e.g., fluorine) is used to remove the deposit on the inner surface. The plasma etch-based cleaning method entails supplying a gas to the chamber and applying a continuous radio frequency (RF) power to form an RF field inside the chamber. The RF field causes the gas to form a plasma. The plasma products (active molecular, atomic, and ionic species) react with the deposit, cleaning the inner surfaces of the chamber. Ideally, the cleaning should remove all the deposit and return the chamber to the pre-deposition state. The necessary conditions are that (1) all reaction products are volatile and can be readily pumped out from the deposition chamber, and (2) the etching plasma does not react with chamber materialsxe2x80x94in other words, chamber materials are corrosion-resistant. In reality, none of these conditions are perfectly satisfied. Some products of film deposit removal may form a solid state residue. Additionally, reactive species in etching plasma may react with the chamber materials, resulting in surface corrosion.
Post-clean residue is often referred to herein as buildup. The amount of buildup on a specific inner surface of the CVD chamber depends on the temperature of the surface and the exposure of the surface to ion bombardment during the plasma clean. Typically, the hotter the surface and the higher the ion flux to the surface, the more buildup is created. One part of a CVD chamber which is particularly sensitive to buildup is a deposition site or station, often a showerhead, that is, a special gas injection system that delivers feedstock gases to the chamber and serves as the RF electrode. High station or showerhead temperature and bombardment of the station or showerhead area by ions in the plasma contribute to station or showerhead buildup.
The quality of the films produced in the chamber degrades with each clean cycle. For example, both the regularity in film thickness among different wafers and the uniformity of the film thickness within a single wafer decrease with each successive clean cycle. The film thickness regularity and uniformity are especially sensitive to buildup on and around the stations or showerheads.
FIG. 1 depicts variations in TEOS (tetraethoxysilane)-based oxide film thickness among wafers processed in a chamber that is cleaned by a continuous-RF plasma-etch method. The closeness in film thickness between different wafers is herein referred to as xe2x80x9cfilm thickness regularity.xe2x80x9d In the exemplary process depicted in FIG. 1, the chamber is cleaned after every 50 wafers, as indicated by markers 10. At the beginning, the thickness variation is only about 600 xc3x85, from about 17,000 xc3x85 to about 17,600 xc3x85. However, the film thickness regularity and the overall film thickness decreases with each successive clean cycle. For example, when 500 or more wafers are processed in the chamber (i.e., wafer #500 and wafer #550), the film thickness drifts to a range from about 16,200 xc3x85 to about 17,500 xc3x85. This drift in film thickness indicates not only a general thinning of the film but also an increase in film thickness variation to about 1,300 xc3x85, which is more than twice the initial variation of about 600 xc3x85. The film thickness regularity declines even though the chamber is plasma-cleaned after every 50 wafers.
FIG. 2 depicts variations in TEOS film uniformity within a single wafer processed in a chamber that is cleaned by the continuous-RF plasma-etch method. Film xe2x80x9cuniformity,xe2x80x9d as used herein, indicates how consistent the thickness of a film is within a single wafer. xe2x80x9cDeviation,xe2x80x9d as used herein, refers to the standard deviation divided by an average, and xe2x80x9cPercent Deviationxe2x80x9d, as used herein, refers to Deviation multiplied by 100. The chamber is plasma-cleaned after every 50 wafers, as indicated by markers 20. Initially, for example, from wafer #51 to wafer #200, the film thickness has percent deviation of from about 1.2% to about 1.4% of the average thickness. However, when the chamber is used for 500 or more wafers, for example, from wafer #500 to wafer #550, the percent deviation increases to about 1.8%.
Further development of cleaning processes is desirable.
The present invention is directed to a method and an apparatus for cleaning the inner surfaces of a deposition chamber, especially in the area of a deposition station, such as a dome-type gas injection system with gas injection tubes, for example, that of the Speed(copyright) HDP CVD reactor of Novellus Systems, Inc., a showerhead-type gas injection system, or the like. In accordance with one embodiment, a fluorine-containing source gas, such as NF3 or C2F6, for example, is supplied to the deposition chamber and radio frequency (RF) power is applied to form an RF field inside the chamber. The RF power is modulated from a maximum value (a xe2x80x9cplasma-ON periodxe2x80x9d) to a minimum value (a xe2x80x9cplasma-OFF periodxe2x80x9d) at a frequency from about 100 Hz to about 50 kHz. Typically, the modulation frequency is selected from a range of about 1 kHz to about 50 kHz, the most common value being from about 10 kHz to about 50 kHz. The maximum value of the RF power applied to one deposition station or showerhead may be, for example, from about 300 to about 1,500 Watts in case of a system designed to process 200 mm-diameter wafers. In a larger system, such as a system having a CVD chamber designed to process 300 mm-diameter wafers, a higher maximum power, such as from about 500 W to about 2,500 W, may be applied to a single deposition station or showerhead. The minimum value of the RF power is typically zero.
When RF power is applied to the chamber, such as during a plasma-ON period, fluorine atoms and various ions are formed. The fluorine atoms that come to the deposition station or showerhead surface, such as the surface of a dome, a pedestal, a showerhead, or the like, react with and remove the deposit. Ions bombard the station or showerhead surfaces only during the plasma-ON period. Ions bombard the station or showerhead surfaces only during the plasma-ON period. During the plasma-Off period, ion bombardment of station or showerhead surfaces ceases while reactions with fluorine atoms continue. This is because the decay time of charged species, including ions, is much shorter than the decay time of fluorine atoms due to surface reactions.
RF modulation is believed to decrease post-clean buildup by creating a desirable ratio of fluorine atoms to ions inside the chamber. The desirable ratio of fluorine atoms to ions in a modulated RF system is a combined effect of at least two factors, namely, the cessation of ion bombardment during a plasma-OFF period and the continued activity of fluorine atoms during a plasma-OFF period. That is, during a plasma-OFF period, ion bombardment of the showerhead area ceases even though the fluorine atoms formed during a plasma-ON period continue to remove film deposit in the chamber. Thus, including a plasma-OFF period in the clean process results in decreased average ion bombardment and continued flourine atom activity relative to a continuous-RF process having the same average RF power. Therefore, modulation of RF power from a plasma-ON period to a plasma-OFF period results in an overall decreased ion bombardment relative to a continuous-RF process, ultimately leading to a plasma action that cleans the deposition station or showerhead area effectively with reduced or minimal damage to the stations or showerheads.
In the present invention, various parameters, such as chamber pressure and temperature, gas composition, and gas flow rates, may be adjusted to maintain an optimal concentration of fluorine atoms and ions. In general, a high production rate of fluorine atoms and a low ion bombardment are desirable.
The modulated-RF clean process of the present invention achieves superior cleaning results, such as superior cleaning of the deposition station or showerhead area. As buildup on and around the station or showerhead area adversely affects the uniformity and regularity of films deposited on wafers, implementation of the modulated-RF process may result in improved films and may lengthen the life span of a deposition chamber and a deposition station or showerhead. By way of example, implementation of the modulated-RF process may lengthen periods between openings of the chamber to remove deposition station or showerhead buildup via mechanical wiping.